Quadrature hybrids designed for use below 500 MHz are typically employed in IF processing circuitry. Although these hybrids are found in a wide variety of applications, their full potential has not been realized because of their large imbalance in output amplitude, and in many cases, their relatively narrow bandwidth.
Although the principles presented herein may be applied to different types of hybrids, the specific class of hybrid which will be considered for illustrative purposes is the 3 dB quadrature hybrid also referred to as the 3 dB quadrature coupler. These hybrids are fundamentally four port devices that accept a signal at an input port, divide the signal in half internally and then supply the divided signal to two output ports. In an ideal quadrature hybrid, the difference in the phase angle between the output ports remains at 90 degrees and the amplitude of the output signals remain equal across the useful bandwidth of the device. There is essentially no output from the fourth port as it is isolated from the input port, and in many instances, such as in the systems illustrated herein, this port is terminated internally.
In a practical, currently available hybrid, the difference in phase angle between the outputs of the hybrid over its useful bandwidth does in fact approach the ideal of 90 degrees; however, the output amplitudes do not approach the ideal of remaining equal. Typically, the outputs differ by as much as 1.0 to 1.5 dB. In most applications, it is this imbalance in amplitude, rather than the difference in phase angle that limits the usefulness of the hybrid.
In conventional hybrids, there are large amplitude imbalances at midband and even larger imbalances at the band edges. Since the phase angle remains at nearly 90 degrees, it is these large imbalances at the band edges that determines the usable bandwidth of conventional hybrids. In many cases, the imbalance results in a relatively narrow bandwidth. Table 1 shows the bandwidth and amplitude imbalance of three conventional hybrids.
TABLE 1 ______________________________________ FREQ RANGE AMPLITUDE BAND- IMBALANCE DEVICE WIDTH TYPICAL MAXIMUM ______________________________________ CONVENTIONAL 1 30-50 MHz 1.0 dB 1.5 dB CONVENTIONAL 2 40-70 1.0 1.5 CONVENTIONAL 3 55-90 .8 1.2 ______________________________________
FIG. 1A is a graph of the amplitude response of a conventional hybrid. It can be seen from this Figure that the amplitude imbalance of this conventional hybrid exceeds 1.4 dB at 30 MHz. Because of the large imbalance at the band edges and in particular at 30 and 90 MHz, the usable bandwidth of this hybrid is considered to be only 40 to 80 MHz. It should be noted that the bandwidth of the amplitude response shown in FIG. 1 is an octave which is relatively wide. Many conventional hybrids in this frequency range have bandwidths that are only 10 percent of their center frequency.
There are two types of special multi-octave quadrature hybrid that are currently available; however, these are not available from many sources and each has serious drawbacks. The first type is fabricated in stripline using multisection couplers. This type of hybrid is generally not available below 100 MHz and, even at that relatively high IF frequency, it is quite large, occupying more than 24 square inches of board space. The maximum amplitude imbalance is 2 dB and there are three bandpass ripples. The smallest ripple results in an imbalance of one dB.
The second type of multioctave quadrature hybrid that is currently available is fabricated from ferrite cores. It has a maximum amplitude imbalance of 1.3 dB and an insertion loss of 4 dB. In addition, it also has a passband slope of over 1 dB with four ripples creating amplitude imbalances which approach one dB. The area required for mounting is less that of the stripline type, but is still typically a square inch or more. Neither type of multioctave hybrid is capable of providing the low amplitude imbalance provided by the present invention which is in the order of 0.2 dB or less.